Solid state image pickup device and method of producing solid state image pickup device

ABSTRACT

Forming a back-illuminated type CMOS image sensor, includes process for formation of a registration mark on the wiring side of a silicon substrate during formation of an active region or a gate electrode. A silicide film using an active region may also be used for the registration mark. Thereafter, the registration mark is read from the back side by use of red light or near infrared rays, and registration of the stepper is accomplished. It is also possible to form a registration mark in a silicon oxide film on the back side (illuminated side) in registry with the registration mark on the wiring side, and to achieve the desired registration by use of the registration mark thus formed.

The subject matter of application Ser. No. 10/954,788, is incorporatedherein by reference. This application is a continuation of U.S. Ser. No.10/954,788, filed Sep. 30, 2004, which is a Divisional application U.S.Ser. No. 10/390,845, filed Mar. 17, 2003, now U.S. Pat. No. 6,821,809which claims priority to Japanese Patent Application NumberJP2002-076081 dated Mar. 19, 2002, which is incorporated herein byreference.

The present invention relates to a Solid State Image Pickup Device and amethod of producing a solid state image pickup device for use as variouskinds of image sensors and camera modules.

RELATED ART

In recent years, the demand for video cameras and electronic cameras hasincreased dramatically and these cameras use a CCD type or amplificationtype solid state image pickup device.

Among these, the amplification type solid state image pickup device(CMOS image sensor) includes on a single semiconductor chip an imagepickup pixel portion comprising a plurality of pixels arranged in atwo-dimensional form, and a peripheral circuit portion disposed on theoutside of the image pickup pixel portion. In each pixel of the imagepickup pixel portion, floating diffusion (FD) portion as well as variousMOS transistors including a transfer transistor and an amplificationtransistor are typically provided. In this case, light incident on eachpixel is subjected to photo-electric conversion by a photodiode togenerate a signal charge, the signal charge is transferred to the FDportion by the transfer transistor, the variation of potential at the FDportion is detected by the amplification transistor, and the detectedvariation is converted into an electric signal and amplified, wherebysignals from each pixel are output through signal wires to theperipheral circuit portion.

In addition, the peripheral circuit portion is provided with a signalprocessing circuit for applying a predetermined signal processing, forexample, CDS (correlative double sampling), gain control, A/Dconversion, etc. to the pixel signals from the image pickup pixelportion, and a driving control circuit for controlling the output of thepixel signals by driving each pixel in the image pickup pixel portion,for example, vertical and horizontal scanners, a timing generator (TG),etc.

FIG. 11 is a sectional view showing a device structure in a related-artCMOS image sensor, and shows the structure of one pixel 10 in the imagepickup pixel portion and one MOS transistor 20 provided in theperipheral circuit portion.

The pixel 10 in the image pickup pixel portion includes a P type wellregion 11 on an N type silicon substrate 1, and a photodiode 12 and anFD portion 13 are provided there. A polysilicon transfer electrode 14for transfer gate for transferring a signal charge from the photodiode12 to the FD portion 13 is provided in an upper insulating layer 2 ofthe N type silicon substrate 1, metallic wirings 15 and 16 formed ofaluminum or the like are provided on the upper side of the polysilicontransfer electrode 14, and, further, a light-shielding film 17 having alight receiving opening portion for the photodiode 12 is provided on theupper side of the metallic wirings 15 and 16.

In addition, a passivation film 3 comprised of a silicon nitride film orthe like is provided on the upper insulating layer 2, and an on-chipcolor filter 28 and an on-chip micro-lens 19 are provided on the upperside of the passivation film 3.

On the other hand, the MOS transistor 20 in the peripheral circuitportion is provided with a P type well region 21 on the N type siliconsubstrate 1, and a source region 22 and a drain region 23 are providedthere. The upper insulating layer 2 of the N type silicon substrate 1 isprovided with a polysilicon gate electrode 24 of the MOS transistor 20,metallic wirings 25, 26 and 27 formed of aluminum or the like areprovided on the upper side of the polysilicon gate electrode 24, and,further, a metallic wiring 28 formed of aluminum or the like is providedalso in the passivation film 3 on the upper side of the metallicwirings.

In the solid state image pickup device as described above, each pixel isso constructed that in order to enhance the numerical aperture of thephotodiode 12 (the ratio of the incident light on the photodiode 12 tothe incident light on the pixel), the incident light is condensed on thephotodiode 12 through the portion between the wirings by a micro-lens19.

In this case, however, a part of the light condensed by the micro-lens19 is repelled by the wirings 15 and 16. This causes the followingundesired problems.

(1) Sensitivity is lowered as much as the amount of the light repelledby the wirings.

(2) A part of the light repelled by the wirings enters into thephotodiode of the adjacent pixel, resulting in mixture of colors.

(3) Since the layout of the wirings is restricted, characteristics arelowered by the limitations such that the wirings cannot be located onthe upper side of the photodiode, or thick wirings cannot be utilized:

(4) Miniaturization is difficult to achieve for the same reason as (3)above.

(5) Since skew incidence of light occurs and the proportion of the lightrepelled is higher at the pixels in a peripheral area, dark shadingoccurs more heavily in the peripheral area.

(6) When it is intended to produce the CMOS image sensor by an advancedCMOS process in which the number of the wiring layers is increasedfurther, the distance from the micro-lens to the photodiode is enlarged,and the above difficulties are further increased.

(7) Due to (6) above, the typical advanced CMOS processing techniquescannot be used, correction of the layout of the circuits registered inthe library is needed, or the number of the wiring layers is limited andtherefore the area is enlarged, so that the cost is raised. Besides, thepixel area per pixel is also increased.

In addition, when long-wavelength light such as red light is subjectedto photo-electric conversion in the P type well region 11 deeper thanthe photodiode 12, the electrons generated diffuse into the P type wellregion 11, resulting in that the electrons enter into the photodiode 12located at another position to cause mixing of colors. When theelectrons enter into a pixel light-shielded for detection of black, theblack level is detected erroneously.

Besides, while there is a process in which a silicide is used for theactive region, the silicide hampers the incidence of light, so that aprocess of removing only the silicide on the photodiode 12 must beadded.

Therefore, the number of steps is increased, and the process becomescomplicated. In addition, defects in the photodiode arise from thesteps, also.

Furthermore, such functions as a camera signal processing circuit and aDSP which have hitherto been composed of other chips are mounted on theperipheral circuit portion of the CMOS image sensor, as described above.As to these, since the process generation is advanced in the manner of0.4 μm→0.25 μm→0.18 μm→0.13 μm, the CMOS image sensor itself must bemade to correspond to these new processes; if it is not fulfilled, themerits of miniaturization cannot be offered and the abundant library andknowledge of CMOS circuits cannot be utilized.

However, the number of layers in the wiring structure increases as theprocess generation advances. For example, while three wiring layers areused in the 0.4 μm process, eight wiring layers are used in the 0.13 μmprocess. In addition, the thickness of the wiring layer is alsoincreased, and the distance from the micro-lens to the light receivingsurface of the photodiode is increased by a factor of 3 to 5.

Therefore, in the related-art method in which the light is passed to thelight-receiving surface through the wiring layers, it has come to beimpossible to efficiently condense the light onto the light-receivingsurface of the pixel, and the problems of (1) to (7) above have come tobe conspicuous.

Meanwhile, recently, the so-called back-illuminated type solid stateimage pickup device in which the light-receiving surface of thephotodiode is provided on the back side of a semiconductor chip has beenproposed as a solid state image pickup device other than theabove-mentioned CMOS image sensor.

This device is constructed as a frame transfer type CCD image pickupdevice, in which a silicon substrate is made to be a thin film, transferelectrodes and the like are provided on the face side of the thin film,and the light-receiving surface of the photodiode is disposed on theback side.

Then, the light received by the light-receiving surface is subjected tophoto-electric conversion by the photodiode in the silicon substrate,and signal charges are trapped by a depletion layer extending from thesubstrate face side, are accumulated in a potential well (P+ type wellregion) on the face side, and transferred and outputted.

FIG. 12 is a sectional view showing the device structure of a photodiodeportion in such a back-illuminated type solid state image pickup device.

The solid state image pickup device has a structure in which anepitaxially grown N type well region 31 is provided on a thin film typeP− type silicon substrate 30, and a P+ type well region 33 is providedon the upper side of the N type well region 31 through a depletion layer32 therebetween to compose a photodiode.

Moreover, an oxide film 34 and an aluminum light-shielding film 35 areprovided on the P+ type well region 33.

In addition, the side of the P− type silicon substrate 30 is the backside, i.e., the light-illuminated side, and the side of the oxide film34 and the aluminum light-shielding film 35 is the face side, wherewirings for transfer electrodes, for example, and the like are disposed.

However, the image pickup device with such a structure has the problemthe sensitivity for blue color for which absorptivity is high islowered. In addition, since light is incident on the back side and issubjected to photo-electric conversion at a shallow position, the signalcharges generated diffuse and, in a proportion, would enter into thephotodiodes in the surroundings.

On the other hand, in the case of the CCD type image pickup device, itis needless to enlarge the height of the wiring layer sincesystem-on-chip is not adopted, and the light-shielding film can bedropped into the surroundings of the photodiode since a process peculiarto CCD is adopted, so that condensation of light by an on-chip lens iseasy to achieve. Therefore, in the case of the CCD type image pickupdevice, the above-mentioned problems (1) to (7) encountered in the caseof the CMOS image sensor are not generated.

From the above circumstances, the back-illuminated type CCD type imagepickup device has almost not been put to practical use, and such aback-illuminated type CCD image pickup device in which color filters andmicro-lenses are in an on-chip form are not typically utilized.

In contrast, in the case of the CMOS image sensor, a process obtained byslight correction to a standard CMOS process is used. Therefore, theCMOS image sensor has the merit which is not possessed by the CCD typeimage pickup device, such that, by adopting the above-mentionedback-illuminated type, a newest process can always be used without beinginfluenced by the wiring step.

In addition, the structure in which a number of layers of metallicwirings extend in crossing directions is absent in the case of the CCDtype image pickup device. Therefore, different from the case of the CCDtype image pickup device, the above-mentioned problems (1) to (7) areconspicuous particularly in the case of the CMOS image sensor. From thispoint of view, also, adoption of the back-illuminated type for the CMOSimage sensor is advantageous.

However, on one hand, at the time of forming color filters and on-chipmicro-lenses on a wafer of an ordinary CMOS image sensor, registration(positioning) of a stepper is conducted by use of the metallic wiringlayer formed of aluminum or the like. On the other hand, at the time ofproducing the back-illuminated type CMOS image sensor, after the wiringstep for the wafer is completed, the wafer is inverted face side back,the side opposite to the side where the wiring is provided is polished,then formation of a silicon oxide film (SiO2), formation of alight-shielding film and formation of a passivation film are conducted,and thereafter formation of back-side color filters and back-sidemicro-lenses is conducted.

Therefore, in the case of producing the back-illuminated type CMOS imagesensor, there is the problem that the registration mark formed at thetime of producing the aluminum wiring layer cannot be used as it is asin a related art.

Accordingly, it is an object of the present invention is to provide amethod of producing a solid state image pickup device in which variouskinds of registrations at the time of producing the so-calledback-illuminated type amplification type solid state image pickup device(CMOS image sensor) can be provided easily and appropriately, andproduction efficiency and device accuracy can be improved.

SUMMARY OF THE INVENTION

In order to attain the above object, according to the present invention,there is provided a method of producing a solid state image pickupdevice comprising a semiconductor substrate provided with an imagepickup pixel portion in which a plurality of pixels each comprising aphoto-electric conversion device and a field effect transistor arearranged in a two-dimensional array. A peripheral circuit portioncomprising a driving circuit for driving the image pickup pixel portionand a signal processing circuit for processing a pixel signal outputtedfrom the image pickup pixel portion, and a wiring layer, are alsoprovided on a first side of the semiconductor substrate. The peripheraldrive circuit is used for driving the field effect transistors in theimage pickup pixel portion. A light-receiving surface of thephoto-electric conversion device is formed on a second side of thesemiconductor substrate, wherein a registration mark is formed by use ofan active region or a gate layer for the field effect transistorsarranged on the first side of the semiconductor substrate, andregistration of each device on the second side in the subsequent step isachieved by use of the registration mark.

In accordance with the method of producing a solid state image pickupdevice of the present invention, the registration mark is formed byusing the active region or the gate layer for the field effecttransistors arranged on the wiring side (first side) opposite to theilluminated side of the semiconductor substrate, whereby theregistration for each device on the second side which has been difficultto achieve by use of a metallic wiring layer as in the related art canbe achieved by detecting the registration mark formed on the first sideof the semiconductor substrate through the thin film semiconductorsubstrate.

Therefore, the desired positioning and registration for each device canbe conducted easily and appropriately, without applying any specialregistration means to the second side of the semiconductor substrate,and production efficiency and device accuracy can be significantlyimproved.

Also, there is provided a solid state image pickup device comprising asubstrate, a first alignment mark formed on a first surface of saidsubstrate, a micro-lens formed on a second surface of said substrate,wherein said alignment mark is formed only on a surface portion of saidfirst surface, and wherein a position of said micro-lens has a constantrelationship with that of said alignment mark.

Further, there is provided a solid state image pickup device comprisinga photo-electric conversion region, a MOS transistor formed on a firstsurface of a substrate, wherein said photo-electric conversion region iscomprised of a first impurity region of a first conductivity type formedon a second surface of said substrate, a second impurity region of asecond conductivity type formed on said first impurity region and athird impurity region of said first conductivity type formed on saidsecond impurity region, and wherein said MOS transistor is comprised ofa first well of said first conductivity type and a source and a drainregions of said second conductivity type formed in said first well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—is a plan view showing schematically an outline of aback-illuminated type CMOS image sensor according to an embodiment ofthe present invention;

FIG. 2—is an equivalent circuit diagram showing the constitution of apixel in the back-illuminated type CMOS image sensor shown in FIG. 1;

FIG. 3—is a general plan view showing a concrete example of pixel layoutin the back-illuminated type CMOS image sensor shown in FIG. 1;

FIG. 4—is a general plan view showing a concrete example of pixel layoutin the back-illuminated type CMOS image sensor shown in FIG. 1;

FIG. 5—is a sectional view showing the device structure in theback-illuminated type CMOS image sensor shown in FIG. 1;

FIG. 6—is a sectional view showing somewhat in detail the devicestructure in the back-illuminated type CMOS image sensor shown in FIG.1;

FIGS. 7(A), 7(B), and 7(C)—show sectional views showing a productionprocess of the back-illuminated type CMOS image sensor shown in FIG. 1;

FIGS. 8(D), 8(E), and 8(F)—show sectional views showing the productionprocess of the back-illuminated type CMOS image sensor shown in FIG. 1;

FIGS. 9(G) and 9(H)—show sectional views showing the production processof the back-illuminated type CMOS image sensor shown in FIG. 1;

FIGS. 10(I) and 10(J)—show sectional views showing the productionprocess of the back-illuminated type CMOS image sensor shown in FIG. 1;

FIG. 11—is a sectional view showing the device structure in a CMOS imagesensor according to a related art; and

FIG. 12—is a sectional view showing the device structure of a photodiodeportion in the back-illuminated type solid state image pickup deviceshown in FIG. 11.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Now, an exemplary embodiment of the method of producing a solid stateimage pickup device according to the present invention will be describedbelow.

According to the present exemplary embodiment, in the step of producinga back-illuminated type solid state image pickup device for coping witha new generation of process, a registration mark is formed on the wiringside by diverting an active region or a gate electrode (polysiliconfilm) for use in the step of forming a MOS transistor, for example, inorder to conduct registration for a stepper.

In addition, in order to form the registration mark, a silicide filmused at the active region may also be used, and the silicide film may beleft on photodiodes (on the side opposite to the illuminated side).

Thereafter, registration of the stepper is conducted by reading theregistration mark with red light or near infrared rays from the backside.

Incidentally, it is possible to form a registration mark on a siliconoxide film on the back side (illuminated side) in conformity with theregistration mark on the wiring side and to conduct registration by useof this mark.

Accordingly, the back-illuminated type amplification type solid stateimage pickup device (CMOS image sensor) can be easily produced, and theabove-mentioned problems as to condensation of light and the like can besolved.

First, an outline of the CMOS image sensor according to the presentembodiment will be described.

FIG. 1 is a plan view showing schematically the outline of the CMOSimage sensor according to the present embodiment, and FIG. 2 is anequivalent circuit diagram showing the constitution of the pixel in theCMOS image sensor shown in FIG. 1.

The CMOS image sensor according to the exemplary embodiment includes animage pickup pixel portion 112, a V selection means 114, an H selectionmeans 116, a timing generator (TG) 118, an S/H□CDS portion 120, an AGCportion 122, an A/D portion 124, a digital amplifier portion 126 and thelike which are formed on a semiconductor chip 110.

The image pickup pixel portion 112 includes a multiplicity of pixelsarranged in a two-dimensional matrix form. As shown in FIG. 2, eachpixel is provided with a photodiode (PD) 200 which is a photo-electricconversion device for generating a signal charge according to the amountof light received and accumulating the signal charge, and further withfour MOS transistors, namely, a transfer transistor 220 for transferringthe signal charge converted and accumulated by the photodiode 200 to afloating diffusion portion (FD portion) 210, a reset transistor 230 forresetting the voltage of the FD portion 210, an amplification transistor240 for outputting an output signal corresponding to the voltage of theFD portion 210, and a selection (address) transistor 250 for outputtingthe output signal from the amplification transistor 240 to a verticalsignal conductor 260.

In the pixel thus constituted, the signal charge having undergonephoto-electric conversion by the photodiode 200 is transferred to the FDportion 210 by the transfer transistor 220. The FD portion 210 isconnected with a gate of the amplification transistor 240, and theamplification transistor 240 constitutes a source follower together witha fixed current source 270 provided in the outside of the image pickuppixel portion 112. Therefore, when the address transistor 250 is turnedON, a voltage according to the voltage of the FD portion 210 is outputto the vertical signal conductor 260.

In addition, the reset transistor 230 resets the voltage of the FDportion 210 to a fixed voltage (in the embodiment shown, a drivingvoltage Vdd) independent from the signal charge.

Besides, the image pickup pixel portion 112 includes various kinds ofdriving wirings for controlling the driving of each MOS transistor whichare wired in the horizontal direction. The pixels in the image pickuppixel portion 112 are sequentially selected in the vertical direction ona horizontal line (pixel row) basis by the V selection means 114, andthe MOS transistors in the pixels are controlled by various pulsesignals from the timing generator 118, whereby the signals of the pixelsare read by the S/H□CDS portion 120 through the vertical signalconductor 260 on a pixel column basis.

The S/H□CDS portion 120 includes S/H□CDS circuits on the basis of eachpixel column in the image pickup pixel portion 112, and applies signalprocessing such as CDS (correlative double sampling) to the pixelsignals read from each pixel column in the image pickup pixel portion112. In addition, the H selection means 116 outputs pixel signals fromthe S/H□CDS portion 120 to the AGC portion 122.

The AGC portion 122 applies predetermined gain control to the pixelsignal from the S/H□CDS portion 120 selected by the H selection means116, and outputs the pixel signal to the A/D portion 124.

The A/D portion 124 converts the pixel signal from the AGC portion 122from an analog signal to a digital signal, and outputs the digitalsignal to the digital amplifier 126. The digital amplifier 126 performsnecessary amplification and buffering as to the digital signal outputfrom the A/D portion 124, and outputs the resulting signal from anexternal terminal which is not shown.

In addition, the timing generator 118 supplies various timing signalsalso to other portions than the pixels in the image pickup pixel portion112 described above.

FIGS. 3 and 4 are general plan views showing a concrete example of pixellayout in the CMOS image sensor according to the present embodiment.

First, FIG. 3 shows the layout of the active regions (the regions wherea gate oxide film is disposed) of the photodiode and each transistor,gate electrodes (polysilicon film), and contacts therefor.

As shown in the figures, the active region 300 of each pixel iscomprised of a rectangular region 310 comprising the above-mentionedphotodiode (PD) 200 and FD portion 210, and a bent belt form region 320extended in an L shape from one corner of the rectangular region 310.

A contact 311 is provided at the FD portion 210 in the rectangularregion 310, a transfer gate electrode 312 is provided at an intermediatelocation between the photodiode (PD) 200 and the FD portion 210, and acontact 313 is provided at an end portion of the transfer gate electrode312.

A reset gate electrode 321, an amplification gate electrode 322 and anaddress gate electrode 323 are provided in this order in the bent beltform region 320, and contacts 324, 325 and 326 are provided respectivelyat end portions of the gate electrodes 321, 322 and 323. The contact 311for the FD portion 210 and the contact 325 for the amplification gateelectrode 322 are connected to each other by an in-pixel metallicwiring.

In addition, a contact 327 connected to the Vdd for resetting isprovided between the reset gate electrode 321 and the amplification gateelectrode 322, and a contact 328 connected to the vertical signalconductor 260 is provided at an end portion of the bent belt form region320.

In addition, FIG. 4 shows the metallic wirings in the upper layers thanthose in FIG. 3 and contacts between the metallic wirings, together withactive regions. In the present embodiment, the metallic wirings areprovided in three layers, in which the first layer is used as anin-pixel wiring 330, the second layer is used as a wiring 340 in thelongitudinal (vertical) direction, and the third layer is used as awiring 350 in the crosswise (horizontal) direction.

These metallic wirings 330, 340 and 350 have hitherto been disposedaloof from the photodiode regions; here, they are disposed also on theupper side of the photodiodes (namely, on the side opposite to theilluminated side). It is clear that by the related-art wiring method inwhich the wirings are aloof from the photodiodes, the pixels with thesize as shown in the figure cannot be laid out.

FIG. 5 is a sectional view showing the device structure in theback-illuminated type CMOS image sensor according to the presentembodiment, and shows the structure of one pixel 400 in the image pickuppixel portion and one MOS transistor 500 provided in the peripheralcircuit portion. Incidentally, in FIG. 5, the upper side in the figureis the illuminated side (back side), and the lower side is the wiringside (face side).

The CMOS image sensor includes the above-mentioned three metallicwirings 330, 340 and 350 provided in the inside of a silicon oxide filmlayer 610 provided on a substrate support material (glass-resin or thelike) 600, and the above-mentioned pixel 400 and MOS transistor 500 areprovided in a silicon layer (N type silicon substrate) 620 provided onthe silicon oxide film layer 610.

Incidentally, FIG. 5 shows a general constitution, and general points ofthe device structure will be described here; the details will bedescribed later, referring to FIG. 6.

The pixel 400 has a structure in which a photodiode 420 is provided inthe state of piercing through the silicon layer 620, at an intermediateportion between P type well regions 410A and 410B provided in the stateof piercing through the silicon layer 620.

The above-mentioned FD portion 210 is provided in the P type well region410A on one side, and the above-mentioned transfer gate electrode 312 isprovided in the inside of the silicon oxide film layer 610 located at anintermediate position between the photodiode 420 and the FD portion 210.

In addition, the MOS transistor 500 has a structure in which a P typewell region 510 is provided in the region on the silicon oxide filmlayer 610 side of the N type silicon layer 620, source/drain (S/D) 520Aand 520B are provided in the P type well region 510, and a gateelectrode (polysilicon film) 530 is provided on the side of the siliconoxide film layer 610.

In addition, a P+ type region 630 is provided on the N type siliconlayer 620, and a silicon oxide film (SiO2) 640 is provided on the upperside thereof. Further, a light-shielding film 650 formed of aluminum orthe like is provided on the upper side of the silicon oxide film 640,and the light-shielding film 650 is provided with an opening portion650A corresponding to a light-receiving region of the photodiode 420.

Incidentally, though not shown in the figure, a pixel for detection ofblack level is formed in the same device structure as the pixel 400shown in FIG. 5, but the opening portion 650A of the light-shieldingfilm 650 is not formed in the light-receiving region so that a signalcharge in the state of being free of light reception is outputted as ablack level reference signal.

In addition, a silicon nitride film (SiN) 660 as a passivation layer isprovided on the upper side of such a light-shielding film 650, and,further, on the upper side of this, a color filter 670 and a micro-lens680 are disposed in an on-chip structure in a region corresponding tothe image pickup pixel portion.

Incidentally, the wafer constituting such a CMOS image sensor ispolished by CMP (chemical mechanical polishing) so that the portion ofthe silicon layer 620 is reduced in film thickness to about 10 μm, forexample.

In view of frequency characteristics of light, a desirable filmthickness range is 5 to 15 μm for visible rays, 15 to 50 μm for infraredrays, and 3 to 7 μm for ultraviolet region.

Besides, the light-shielding film 650, unlike the wirings, can be laidout by taking only optical elements into account. The metallic layerpresent in the range from the micro-lens 680 to the photodiode 420 isonly the light-shielding film 650, and the height of the light-shieldingfilm 650 from the photodiode 420 is as small as the thickness of thesilicon oxide film 640, for example, about 0.5 μm; therefore, unlike theabove-mentioned related-art example, the restrictions on lightcondensation due to repelling by the metallic wirings can be obviated.

FIG. 6 is a sectional view showing somewhat in detail the well structurein the above-mentioned N type silicon layer 620. Incidentally, in thedevice structure shown in FIG. 6, the upper side of the figure is thewiring side (face side) and the lower side is the illuminated side (backside), oppositely to FIG. 5. In addition, the elements common in bothFIGS. 5 and 6 are denoted by the same symbols as before, and descriptionthereof is omitted.

As for the MOS transistor 500 in the peripheral circuit portion, thesame contents as in FIG. 5 are shown, but a low-concentration N− type isused for the N type silicon layer (silicon substrate) 620 as shown inthe figure.

On the other hand, as for the pixel 400 in the image pickup pixelportion, a MOS transistor 430 other than the transfer transistor(namely, in this embodiment, an amplification transistor, a resettransistor, or an address transistor) is shown in addition to thecontents of FIG. 5.

As described above, the pixel 400 has a structure in which deep P typewell regions 410A and 410B are provided in the state of piercing throughthe silicon layer 620, and a photodiode 420 is provided in the state ofpiercing through the silicon layer 620 at an intermediate portiontherebetween.

The photodiode 420 is comprised of a shallow P+ type layer 420A (a partof a P+ type region 630) on the illuminated side, an N− type layer 420B(a part of the silicon layer 620) in the inside thereof, and a deep P−type well region 420C on the wiring side, and an FD portion 210 and atransfer transistor 220 are formed in the P− well region 420C on thewiring side.

In addition, the N− type layer 420B is the photo-electric conversionregion, which has been completely depleted because the area is small andthe concentration is low.

An N+ type region 440 for accumulating signal charges is formed at apart of a boundary portion between the N− type layer 420B and the P−type well region 420C. In addition, a P+ type region 450 for forming anembedded photodiode is provided on the wiring side, adjacently to the N+type region 440.

The signal charge is transferred to an N+ type region of the FD portion210 by an action of the transfer transistor 220. In addition, under thecondition where the transfer transistor 220 is OFF, the N+ type regionson the photodiode 420 side and on the FD portion 210 side areelectrically separated from each other by the P− type well region 420Cpresent therebetween.

Besides, the MOS transistor 430 other than the transfer transistor 220is formed in the deep P type well region 410A as usual, and has astructure in which N+ type source/drain regions 431 and 432 are formedin the P type well region 410A, and a gate electrode 433 is formed onthe upper side thereof.

Next, a method of producing the CMOS image sensor constituted as abovewill be described.

FIGS. 7 to 10 are sectional views showing the process for producing theCMOS image sensor according to this embodiment.

(1) Device Separation, and Formation of Wells

First, device separation regions and various well regions are formed ina silicon substrate (silicon layer 630) before being converted into athin film. Here, as above-mentioned, a deep P type well region is formedin the pixel portion, and a shallow P type well, region and an N typewell region are formed in the peripheral circuit portion.

(2) Formation of Various Transistors, Wirings, and Pads

As shown in FIGS. 7(A) and 7(B), various MOS transistors, aluminumwirings, electrode pads and the like are formed in steps similar to therelated-art process for producing a CMOS image sensor. In the presentembodiment, a registration mark for a stepper is formed by use of a gateor an active region of the MOS transistor.

Incidentally, as a proposal precedent to the present application, therehas been proposed a method in which, for conducting registration of thestepper on the back side in the subsequent step, a trench (groove) isformed in the wafer at this stage, and tungsten, aluminum or the like isdeposited into the trench to form a mark. In this method, theregistration mark can be formed at a deep position in the substrate, ora position near to the back side, but impurities such as metallic atomswould easily enter into the substrate at the mark location. In thatcase, a defect would be generated in the pixel with a certainprobability, and a white spot would appear in an image picked up by thesolid state image pickup device. Obviously, these results areundesirable and the previously proposed solution is not practical.

In view of this, in the present embodiment, a registration mark 700 isformed by diverting a gate electrode (polysilicon) or an active regionformed for the MOS transistor. Particularly in the active region, it ispreferable to form the registration mark of a silicide (a compound of ametal and silicon) such as cobalt silicide.

Besides, in this case, the silicide film can be left on the photodiodes(the side opposite to the illuminated side). Accordingly, a step ofremoving the silicide film can be omitted, and steps can be simplified.In addition, the defects (appearance of white spots on the picked-uppicture) due to the removal step can be prevented.

Besides, it is possible to prevent the problem that the light incidentfrom the back side is transmitted through the photodiode and reflectedby the wirings, to be subjected to photo-electric conversion at otherphotodiodes.

(3) Adhesion of Substrate Support Material

As shown in FIG. 7(C), a glass material is made to flow onto the wiringside, to form a substrate support material (more in detail, a firstlayer of the substrate support material) 600A. Incidentally, in thiscase, a resist 710 is patterned on the position where a pad 722 is to beformed.

(4) Formation of Pad and Contact

As shown in FIG. 8(D), to form a contact, the resist 710 is removed, ahole 711 is opened in the substrate support material 600A, and a surfacetreatment is conducted, to expose a bump for connection. Then, as shownin FIG. 8(E), a metal for contact is introduced to the hole 711 and thesurface of the substrate support material 600A, to form the contact 720,and, as shown in FIG. 8(F), the metallic film on the surface of thesubstrate support material 600A is patterned, to form an electrode pad721.

Thereafter, as shown in FIG. 9(G), a second layer of substrate supportmaterial 600B for flattening on the wiring side is made to flow onto thefirst layer of substrate support material 600A, followed by polishing.

(5) Back Side Polishing

Thereafter, the wafer is inverted face side back, and the back side ispolished by CMP until the film thickness of the silicon layer 630becomes about 10 μm.

(6) Formation of Back-Side Silicon Oxide Film

For example, by CVD (chemical vapor deposition), a thin silicon oxidefilm (SiO2) 640A (a part of the silicon oxide film 640) is formed in afilm thickness of about 10 nm, for example.

Here, as shown in FIG. 9(G), in registry with the registration mark 700formed of the gate layer or the silicide-added active region formed onthe wiring layer side, a registration mark 730 is formed in theback-side silicon oxide film 640. This is formed by etching, in themanner of slightly grinding, the silicon oxide film 640A to the siliconlayer 630.

Incidentally, the formation of this back-side registration mark 730 isnot indispensable, as will be described later.

(7) Back-Side P+ Implantation

Next, boron in such an amount as to fill up the interface of the siliconoxide film with positive holes is added by ion implantation through thesilicon oxide film 640.

Incidentally, in the proposal precedent to the present application, theregistration of the stepper has been conducted by use of a registrationmark consisting of the trench preliminarily formed in the surface of thewafer as described above. In the present exemplary embodiment,registration can be conducted by either of the following methods.

“A”: The registration mark 700 of the gate layer or the active regionformed in (2) above is used.

“B”: The registration mark 730 formed in the silicon oxide film in (6)above is used.

Therefore, in the case of using the method “A”, the formation of theregistration mark 730 in “B” can be omitted.

Incidentally, in detecting the registration mark 700 on the wiring side(face side) by the method “A”, use of red light or near infrared rayswith a wavelength of 0.61 to 1.5 μm makes it possible to enhance thedetection efficiency.

(8) Formation of Back-Side Silicon Oxide Film

Next, the residual silicon oxide film 640B is formed by CVD, in a filmthickness of 500 nm, for example.

(9) Formation of Back-Side Light-Shielding Film

Subsequently, a light-shielding film 650 of aluminum, tungsten or thelike is formed by the CMOS process.

In this case, registration is conducted by the method “A” or “B”described in (7) above. Here, a registration mark (not shown) for acolor filter and a micro-lens which will be formed in the subsequentstep is formed.

(10) Formation of Passivation Film

A plasma SiN film 660 is formed by CVD (FIG. 9(H)).

(11) Formation of Color Filter 670 and Micro-Lens 680 (OCL) (FIG. 10(I))

The steps (10) and (11) above are conducted by the same method as in therelated art.

However, the stepper registration is conducted by use of the mark formedin (9). In addition, in the case where the light-shielding film is notused, it is conducted by use of the method “A” or “B” described in (7).

(12) Exposure of Pad Surface

Next, as shown in FIG. 10(J), the second layer of substrate supportmaterial 600B on the electrode pad 721 mentioned above is removed byetching, to expose the electrode pad 721. In this case, the second layerof substrate support material 600B for, for example, registration of themicro-lens and flattening of the device chip is polished to adjust thethickness thereof to a predetermined thickness. Besides, since theportion of the electrode pad 721 is present on the side opposite to thelight-receiving side, direct mounting onto the substrate is possible.

As has been described above, in the method of producing a CMOS imagesensor according to the present exemplary embodiment, the register markis formed on the wiring layer side of the silicon substrate by use ofthe gate layer or the active region, and the registration mark is usedfor registration of the light-shielding film or the color filter or theon-chip lens on the back side, or the register mark is formed on theback side based on the gate layer or the silicide-containing activeregion on the wiring layer side, and the registration mark is used forpositioning or registration of the light-shielding film or the colorfilter or the on-chip lens on the back side.

Therefore, since it is unnecessary to form the registration mark for theback side in a special step, the steps are simplified, and it ispossible to prevent the problem that impurities such as metallic atomswould enter into the substrate through the registration mark portion togenerate defects.

In addition, particularly by forming the active region of a silicidesuch as cobalt silicide, it becomes easy to detect the mark from theback side. Besides, in confirming the registration mark on the wiringside from the back side, use of red light or near infrared rays with awavelength of 0.61 to 1.5 μm make it easy to confirm the mark location.

In addition, by not removing the silicide in the active region on thephotodiodes, the number of steps is reduced, the steps are simplified,defects associated with the removal step can be reduced, and, further,it is possible to prevent the problem that the light incident from theback side is transmitted through the photodiode and reflected to besubjected to photo-electric conversion by other photodiodes.

By such a technique, a back-illuminated type CMOS image sensor with lessdefects and good characteristics can be produced by a reduced number ofsteps.

Besides, the back-illuminated type CMOS image sensor produced by thepresent embodiment has the following additional advantages.

First, the photodiodes are made to be capable of receiving visible raysfrom the back side, whereby it becomes unnecessary to provide wiringswhile taking into account the receiving surfaces as in the related art.Therefore, the degree of freedom of the wirings for the pixels isenhanced, and miniaturization of the pixels can be contrived.

In addition, since the photodiodes reach the back side, sensitivity forblue color for which absorptivity is high is enhanced, and sincephoto-electric conversion does not occur at portions deeper than thephotodiodes, mixing of colors and erroneous detection of black level dueto such a photo-electric conversion at deeper portions are obviated.

Besides, since the light-shielding film, the color filter and theon-chip lens can be formed at lower positions than the light receivingsurfaces, the problems of lowering of sensitivity, mixing of colors, andlight reduction at peripheral areas can be solved.

In addition, the CMOS image sensor can be produced by an advanced CMOSprocess with a large number of wiring layers.

Furthermore, since the electrode pads are disposed on the side oppositeto the light receiving surfaces, they can be mounted directly on thesubstrate with the light receiving surfaces directed up.

While the specific exemplary embodiment of the present invention hasbeen described above, this is merely one example of the presentinvention, and various modifications of the present invention arepossible.

For example, the specific numerical values of film thickness and thelike, materials and the like shown in the above-described exemplaryproduction steps are not to be construed as limitations for the presentinvention. In addition, the structure of the solid state image pickupdevice produced is not limited to the above embodiment. For example, thestructure of the pixel is not only the one comprised of four MOStransistors, but may be one comprised of three MOS transistors or may beone comprised of five MOS transistors. Besides, the wiring structure fordriving the pixels and the like are naturally not limited to the aboveembodiment.

As has been described above, according to the method of producing asolid state image pickup device according to the present invention, theregistration mark is formed by use of an active region or a gate layerfor the field effect transistors arranged on the wiring side (firstside) opposite to the illuminated side of the semiconductor substrate,whereby the registration of each device on the second side which isdifficult to achieve by use of the metallic wiring layers in the relatedart can be conducted by detecting the registration mark formed on thefirst side of the semiconductor substrate through the thin filmsemiconductor substrate. Therefore, the registration of each device canbe conducted easily and appropriately, without applying any specialregistration means to the second side of the semiconductor substrate,and production efficiency and device accuracy are improved.

1. A method of producing a solid state image pickup device comprising asemiconductor substrate having an image pickup pixel portion with aplurality of pixels each comprising a photo-electric conversion deviceand a field effect transistor and a peripheral circuit portioncomprising a driving circuit for driving said image pickup pixel portionand a signal processing circuit for processing a pixel signal outputfrom said image pickup pixel portion, a wiring layer for driving saidfield effect transistors in said image pickup pixel portion, said wiringlayer being provided on a first side of said semiconductor substrate,and a light-receiving surface of said photo-electric conversion devicebeing provided on a second side of said semiconductor substrate, whereinthe method comprises the steps of: forming a registration mark duringformation of an active region or a gate layer for said field effecttransistors arranged on said first side of said semiconductor substrate,and thereafter placing one or more devices formed on said second side insubsequent processing with reference to said registration mark; furthercomprising forming a sample and hold circuit for receiving signal outputduring all read-out time.
 2. A method of producing a solid state imagepickup device as set forth in claim 1, further comprising a step offorming a silicide film portion in said active region as saidregistration mark.
 3. A method of producing a solid state image pickupdevice as set forth in claim 2, wherein said silicide film is a silicidefilm comprised of a compound of cobalt and silicon.
 4. A method ofproducing a solid state image pickup device as set forth in claim 2,further comprising completing the manufacture of the device withoutremoval of unrequired portions of the silicide film.
 5. A method ofproducing a solid state image pickup device as set forth in claim 1,further comprising detecting the registration mark by irradiating thedevice with a detection light having a predetermined wavelengthtransmitted on said second side of said semiconductor substrate.